Overall, women are much more behaviorally conservative than men -- they have fewer sex partners, start having sex later, are less likely to do drugs, commit crimes, etc. -- and their greater tolerance is an outgrowth of an ethos of personal restraint that respects and is sympathetic toward the autonomy of others, not necessarily out of a progressive political mindset.
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Maybe it just takes until they have kids for women's greater conservatism on values questions to overide their greater tolerance and swing them over to the GOP side. But their greater conservatism is there from the get go.